In wireless networks, a base station (BS) is usually located near the center of a cell. As a result, the quality of signals received at a mobile station (MS) near an edge of the cell is reduced. This problem can be resolved by decreasing the size of the cell. However, this increases costs because the number of BSs needs to be increased to provide the same service for the same coverage area. In addition, this also may cause higher level of interference to MSs residing at the edge of neighboring cells. An alternative solution uses a less complex relay station (RS), which assists in communications between the BS and the MS. The RS can effectively increase coverage and reliability with decreased transmit power and cost.
The RS can be deployed anywhere in the cell wherein direct communication between the BS and the MS is impaired. The RS can also be deployed temporarily in areas where the number of MSs is expected to increase dramatically for a short time period, e.g., large-scale public events.
The RS can use decode-and-forward (DF) or amplify-and-forward (AF) modes. The DF mode detects and demodulates received signals before retransmitting. The AF mode only amplifies the received signal before retransmitting.
Conventionally, the RS can use different frequencies or times to reduce interference. In a frequency-reuse-relay-station (FRRS), the frequency bands for transmitting and receiving signals are identical. In a frequency-shifted-relay-station (FSRS), the frequency bands are different. While the FRRS increases spectral efficiency, cross-talk interference becomes an issue because the transmit signal power is always greater than the receive signal power. In the FSRS, out-of-band leakage can cause cross-talk interference.
Cross-talk occurs when a transmitted signal interferes with a currently received signal. Cross-talk can be caused by undesired capacitive, inductive, or conductive coupling between the transmit antenna and the colocated receive antenna, or lines and circuits to which the antennas are connected. Cross-talk is often denoted as co-channel interference, and is related to adjacent-channel interference, see Nasr et al., “Performance of an echo canceller and channel estimator for on-channel repeaters in DVB-T/H networks,” IEEE Trans. Broadcasting, vol. 53, no. 3, pp. 609-618, September 2007, and Mazzotti et al., “Performance of an echo canceller based on pseudo-noise training sequences,” Proc. 58th Annual IEEE Broadcast Symposium, October 2008.
Prior art techniques require the RS to transmit dedicated pilot signals, such as pseudo-noise sequences, for estimating the coupling channel between the colocated transmit and receive antennas. The pilot signals change the existing signal structure of the physical layer, leading to incompatibility with legacy standards, and also result in interference at the receiver.
It is desired to perform cross-talk cancellation at the RS that is transparent to current wireless standard, i.e., the structure of the signals at physical layer remains the same at the BS and the MS.